The present invention relates to data set diagnostic systems.
The communication of high-speed data over voiceband telephone lines, for example, requires the use of so-called data sets, or modems. The principal function of a data set is to modulate user-provided data into the passband of the telephone line at the transmitting end and demodulate the received data signals out of the passband at the receiving end and recover the user-provided data.
Recently, networks of data sets have been provided with so-called diagnostic capability, in which the data sets and, in some networks, separate diagnostic control devices, communicate with each other via, for example, a narrow bandwidth "secondary" channel within the passband of the telephone line. For example, a control device--such as a diagnostic control device or a control data set--may instruct a "downstream" tributary data set to perform a particular test or to change an option under which the tributary data set operates. The latter, in turn, will perform the action required and return to the upstream control device the results of the test or other indication relative to the requested action.
Typically, a data set receives all the diagnostic communications which orignate upstream of it. To this end, the communication of diagnostic signals over more than one communication link, e.g., from a diagnostic control device to a control data set to a tributary data set, is achieved by providing at each intermediate data set a direct signal path for diagnostic signals to proceed downstream. Each data set downstream of a communications originator examines address information which accompanies the "text" of the communication, e.g., the test or option change instruction itself and acts upon the text only if it is intended for that data set.
A data set diagnostic system architecture of the type described above is generally satisfactory as long as the round trip delays are small. It may be disadvantageous, however, in so-called extended networks in which the diagnostic signals (and, of course, the data signals) must pass through many intermediate data sets. Assume, for example, that a control device at one end of a large extended network transmits a text to a tributary device at the other end. The control device will not know whether the text was correctly received until it has waited for a time interval at least as long as the anticipated round trip delay, which in an extended network can be substantial. Even if the transmission problems occurred in the first link, i.e., in the signal path between the control device and the next data set along the route, the control device would have to wait for that entire time interval before concluding that there was a breakdown in communications, and only then could it initiate a retransmission. Moreover, when the control device retransmits its text, the source of the original transmission error in one link may have abated, but there may now be a problem in a second link, requiring yet another retransmission. As a result of these and other considerations, the time required to execute a particular diagnostic function can become excessive.
A further disadvantage of the above-described type of architecture, which is independent of the delays in the network diagnostic system is that since each data set in the network receives all communications originating upstream, only one diagnostic function can be performed within the network at any given time. This constitutes an inefficient use of the diagnostic facilities.